BOTANY. 



BOTANY. 



observed a layer for every year of age ; these, 

 however, are of little import to the plant, the 

 vital functions for the time being are carried 

 on in the layer immediately in contact with 

 the wood. This innermost ring is termed the 

 liber. The bark is very conspicuous in some 

 roots, as the parsnip, carrot, &c.; the thick 

 outer ring, observable when these are cut 

 transversely, is the* bark. The bark consists 

 of woody fibres, chiefly running longitudinally, 

 but beautifully interwoven. In one of the me- 

 zereon tribe, a native of Jamaica, and called 

 the lace bark, it may be separated into elegant 

 layers of lace-work. In the bark the peculiar 

 properties of the plant principally reside ; wit- 

 ness the resin in the pine, the fragrant oil of 

 the cinnamon, &c. 



Next to the liber occurs the wood, which 

 forms the chief bulk of trees. A layer or more 

 of this occurs in all exogenous plants, for in 

 the portion of it which adjoins the liber, and 

 is named the alburnum, are the sap-vessels 

 which convey the fluid from the root to the 

 leaves, whence it descends into vessels situ- 

 ated in the liber, as we shall see hereafter. In 

 trees, a fresh layer of wood is deposited every 

 year adjoining the liber, from which it is 

 formed or deposited ; hence the age of a tree 

 may be known by counting the concentric 

 rings. In the middle of the wood occurs the 

 medulla or pith, commonly a porous, juicy, yel- 

 lowish or greenish substance; even the hollow 

 stems of the onion, &c., are lined with a film 

 of it. It seems to be an extra reservoir of 

 nourishment, required for the formation of the 

 leaves and more recent parts of plants; at all 

 events, in old stems and branches it is usually 

 obliterated. Botanists are not determined as 

 to its uses. 



When a seed is committed to the ground, 

 if moisture, air, and heat are not all present in 

 certain favourable proportions, it refuses to 

 germinate. (See WATETI, its uses to vegeta- 

 tion.) No seed will vegetate in dry earth, nor 

 in a temperature at or below the freezing 

 point; all require a free admission of air. 

 These circumstances being favourable, the 

 seed swells the skin bursts and the radicle, 

 or embryo root, makes its appearance, and 

 sinks into the earth. The cotyledons, if the 

 seed has more than one, by degrees develope 

 themselves, and rise above the surface, afford- 

 ing nourishment to the embryo stem, situated 

 between them, until the radicle has become 

 sufficiently a root to supply food for its growth; 

 when thus rendered useless, they decay. 



Animal and vegetable matters rendered so- 

 luble in water by putrefaction, various salts 

 and earths, and water, are the chief nourish- 

 ment plants derive from the soil ; but it is also 

 certain that the roots absorb air, which in part 

 accounts for the benefit afforded to them by 

 loosening the soil about them, and for plant- 

 ing them near the surface. When a plant has 

 got its leaves developed, it possesses another 

 source of acquiring nourishment from the at- 

 mosphere. See GASES, their use to vegetation. 



The atmosphere, which to our eyes appears 



a shuple uniform fluid, has been demonstrated 



by chemists to be composed of three different 



gases or airs with which is constantly mixed 



212 



' the vapour of water. The gases are known as 

 I oxygen, carbonic acid, and azote or nitrogen. 

 } Carbonic acid gas is carbon or charcoal com- 

 j bined with oxygen. Water is composed of 

 j hydrogen and oxygen gases. These facts, by 

 a little attention, will be easily remembered, 

 and render all that follows comprehensible. 

 The nourishment which is absorbed by the 

 roots being in a fluid state, proceeds alor.g the 

 sap-vessels situated in the alburnum of the 

 wood, and spreads through the leaves, flowers, 

 &c. Here, and during its course up the stem, 

 by the varied absorption and decomposition of 

 water and carbonic acid, and the emission of 

 oxygen, the sap is converted into various sub- 

 stances, varying in every species of plants ; 

 gum is formed in the cherry, resin in the fir, 

 &c.; these are deposited as the sap descends 

 through the vessels of the liber. From the sftp 

 likewise is derived the nourishment from 

 whence is formed the wood, &c.; in fact, it is 

 the source of the growth of the parts. Our 

 knowledge of chemistry and vegetable physio- 

 logy is yet too imperfect to enable us to mark 

 the various shades of difference in the pro- 

 cesses of each plant with any degree of pre- 

 cision. We know that in the light all plants 

 absorb carbonic acid gas, and emit oxygen 

 whilst in the dark; on the contrary, they ab- 

 sorb the latter and give out the former by the 

 same surfaces ; but we are utterly unable to 

 point out how the same organs secrete a poi- 

 son in the nightshade and a wholesome food 

 in the potato, which so closely resembles the 

 first in form. A few very simple experiments 

 will serve to fix the above facts upon our me- 

 mories. We may prove that the sap rises 

 through the alburnum, and descends through 

 the bark, by placing the cut end of a leafy 

 twig of the fig tree in an infusion of Brazil 

 wood; after some hours cut off about half an 

 inch of the extremity, when a circle of red dots 

 will mark where the infusion ascended, and 

 an outer circle of white dots will show where 

 the juices descend. 



That leaves throw off moisture, or perspire, 

 is demonstrated by inverting a tumbler over two 

 or three leaves placed in the light; the inside 

 of the glass will soon be perceptibly covered 

 with dew. 



That leaves throw off gas from their sur- 

 faces is demonstrated by plunging one in a ves- 

 sel of water; air-bubbles will soon be perceived 

 to be emitted by and attached to it. 



In due course of time the flowers of a plant 

 open; the anthers of the stamens swell, burst, 

 and scatter a dust, termed pollen, secreted by 

 them, and which is caught immediately by the 

 moist stigmas of the pistils, or is carried to 

 them by the wind, or accidental contact of 

 some insect. This contact of the pollen with 

 the stigma is found to be absolutely necessary 

 before the seed can be perfected. This course 

 of vegetation is repeated for a series of years 

 in perennials, but the plant decays as soon as 

 the seed is perfected in annuals. 



Botanists at present are acquainted with 

 nearly 100,000 pecies of plants; and the care 

 with which Providence has provided for me 

 well-being of plants is an earnest of their im 

 portance. That they may never become ex 



